The drilling of earth, as in drilling for oil, typically employs a rotary drilling apparatus including a drill bit at its lower end. The rotary drilling apparatus creates a significant amount of heat due to friction during the drilling process. Therefore, a lubricant or "drilling mud" is employed to reduce heat buildup in the drilling apparatus. In such rotary drilling of wells for oil and gas, drilling fluids are circulated into the well through hollow tubular drill pipe, past the teeth of the cutter head to sweep away the cuttings from the cutter head, and returned to the surface along with the cuttings through the annulus surrounding the drill pipe. The drilling mud is generally circulated in such a manner as to remove drill cuttings to prevent clogging of the cutter and to support the walls of the well hole. Such drill cuttings are entrained in the drilling fluid and brought to the surface with the mud and then screened out and discarded. In addition to removing drill cuttings and supporting the walls of the well hole, the drilling mud also serves to cool and lubricate the drill bit. Since the earth formations encountered during drilling can vary widely, drilling muds are generally selected taking into account the condition of the earth, and types of formations which are encountered during a drilling operation.
Drilling muds normally contain clays and/or other dispersed solids which are employed to impart desired rheological properties to the drilling mud. Not only do these clays and/or other suspended solids impart desirable thixotropic properties to the drilling mud, they also serve to coat the walls of the well with a relatively impermeable sheath, commonly termed a filter cake, which retards the flow of fluid from the well into the surrounding subterranean formations.
In addition to clays and/or other suspended particles, a drilling mud may also contain one or more weighting agents which function to increase the density of the mud to a level which will offset high pressures encountered during the drilling operation. Non-limiting examples of suitable weighting agents include heavy minerals such as barite.
One problem very commonly encountered during rotary drilling operations is the problem of lost circulation in which part or all of the drilling mud is not returned to the surface. This problem may manifest itself anywhere from moderate losses of the drilling mud, to substantial or even total losses of the drilling mud such that little or none of it is returned to the surface. Where a formation zone is identified in which unacceptably large amounts of drilling mud is lost, such formation zone is commonly termed a "thief zone", "loss zone" or a "loss circulation zone." While there are many causes for loss and/or lost circulation, non-limiting examples include those situations when the well encounters a formation of unusually high permeability or one which has naturally occurring horizontal or vertical fractures or fissures. Also, the formation may be fractured accidentally by the hydrostatic pressure exerted by the drilling mud, particularly when a change over to a relatively heavy mud is made in order to control high formation pressures.
Over the years numerous techniques have been developed to prevent or reduce loss and/or lost circulation. One common technique where the loss circulation is not so severe is to add various fluid loss agents which function to change the rheological properties of the drilling mud in order to increase its resistance to flow from the well bore into the formation. Such fluid loss agents include synthetic polymeric thickening agents such as partially hydrolyzed polyacrylamide, polyelectrolytes such as an ionic polysaccharide, various gums such as locust bean gum and guar gum, various starches, and carboxymethyl cellulose (CMC), carboxyethyl cellulose (CEC) hydroxyethyl cellulose (HEC), hydroxypropyl guar, hydroxyethyl guar, etc.
An exemplary drilling mud comprises a gelling agent, such as a solid phase bentonite, a weight agent, such as barite, and water. In addition, salt or salt water can be added to the components of the drilling mud to prepare a salt water based drilling mud. Numerous different additives to this drilling mud are also employed to control viscosity, yield point, gel strength (thixotropic properties), pH, fluid loss, tolerance to contaminants such as salt and calcium carbonate, lubricating properties, filter caking properties, cooling and heat transfer properties, and tolerance to inactive solids such as sand and silt or active native mud making clays such as smectites, illites, kaolinites, chlorites, etc. Those skilled in the art recognize that providing an acceptable drilling fluid in the field requires much labor and high material handling and transportation costs. In addition, environmental and occupational safety considerations restrict the types of additives and their amounts in the drilling muds. Also, selected additives should not have fluorescence properties, so that oil deposits on the drill cuttings can be readily identified.
Over the years, a number of drilling mud formulations have been proposed. For example, U.S. Pat. No. 3,726,850 discloses a lignin dispersing agent for dispersing clays, and the like. The lignin dispersing agent is reported to have utility in both alkaline and acidic media. A relatively low viscosity aqueous silicate solution is disclosed in U.S. Pat. No. 3,746,109, and is reported to be particularly useful in drilling through shale formations. U.S. Pat. No. 4,799,549 discloses a stable, gel-forming microemulsion comprising an aqueous solution of an alkali metal silicate, a gelling reagent, and a surface active agent (surfactant). This composition is reported to be useful for permanent or reversible plugging or clogging of subterranean formations. Also, U.S. Pat. No. 5,374,361 discloses a composition for cleaning out cased wellbores, and the like, using a fluid that includes a caustic alkyl polyglycoside surfactant formulation. This formulation is reported to be more biodegradable than previous detergent systems. A further additive encountered in aqueous drilling fluids is a metal compound, such as that described in U.S. Pat. No. 5,399,548, or a derivative of a metal compound such as a hydroxy-aluminum compound provided in a polymer, such as disclosed in U.S. Pat. No. 4,045,357. U.S. Pat. No. 5,333,698 also discloses a drilling fluid additive in combination with a white non-toxic mineral oil.
Most of the additives discussed above and those employed in the industry are utilized only as additives. That is, they cannot be provided easily, safely, and economically as a "whole mud" system specifically designed to accomplish a given drilling task. Thus, a drilling mud system is desired which can be applied in a modular fashion with one or more modules being used as needed for a particular set of requirements. Such a system should also enable a person skilled in the art to convert an old mud formulation, e.g., salt muds, lime muds, low solid muds, oil muds, synthetic oil mud, etc., into such a modular system.
There is also an urgent need for improved labor health and safety in working with drilling muds, improved cleanliness of rig floor and equipment, environmental safety and non-toxicity of additives, as well as reduced handling and storage costs.
To solve the problems of the prior art, the present inventors developed a modular drilling mud or fluid system. This modular drilling fluid or mud is described U.S. Pat. No. 5,755,295. This modular drilling fluid system can be conveniently formulated as at least one of the following modules:
(1) an alkaline first module that contains a source of caustic, a natural wax, and a natural thinner; PA1 (2) a second module prepared by reacting aluminum metal with the components of the alkaline first module thereby forming a soluble alkali metal aluminate; PA1 (3) a third module containing the components of the first module in combination with at least one of an alkali metal phosphate, alkali metal borate and sodium silicate; PA1 (4) a fourth module containing the components of the first module in combination with a C.sub.1-24 saturated or C.sub.2-24 unsaturated carboxylic acid source, a surfactant, and a preservative; and PA1 (5) a fifth module containing the components of the first module in combination with a carboxylic acid source, a surfactant, a preservative, and at least one of an alkali metal phosphate, alkali metal borate and sodium silicate. PA1 (1) an alkaline first module that contains a source of caustic, a natural wax, and a natural thinner; PA1 (2) a second module prepared by reacting aluminum metal with the components of the alkaline first module thereby forming a soluble alkali metal aluminate; PA1 (3) a third module containing the components of the first module in combination with at least one of an alkali metal phosphate, borate and sodium silicate; PA1 (4) a fourth module containing the components of the first module in combination with a saturated or unsaturated carboxylic acid source, a surfactant, and a preservative; and PA1 (5) a fifth module containing the components of the first module in combination with a saturated or unsaturated carboxylic acid source, a surfactant, a preservative, and at least one of an alkali metal phosphate, borate and sodium silicate. PA1 (1) a boost in the lubricity of the modular drilling mud or fluid system, PA1 (2) a boost in the mud thinning or mud viscosity controlling power of the modular drilling mud or fluid system, PA1 (3) a decrease in the strength of the clay particle bond between the drilled clays and the drill bit, or a decrease in bit "balling" tendencies, PA1 (4) a decrease in the rock cohesive or shearing resistance immediately beneath the drill bit, or work front due to penetration of low surface tension dispersants like borates, phosphates and the like into the clay bonds and reduction in capillary entrance pressure due to low surface tension(5 to 35 dynes/cm or the present formulation, and PA1 (5) control of phase separation of the modular drilling mud or fluid system and control of the excessive loss of the mud filtrate to which the modular drilling mud or fluid system is added.
It has further been discovered that a combination of glycerol and phosphoric and/or boric acid added to any one of the forgoing modules can further improve some properties of a drilling mud in which the module is added. The improved properties include lubricity of the drilling mud, mud thinning and mud viscosity controlling, decrease in the bit "balling" tendencies, decrease in the cohesive or shearing resistance beneath the drill bit and improved control of phase separation of the mud. This modified modular drilling fluid, mud and associated methods of using the modified fluid and mud are the subject U.S. patent application Ser. No. 09/083,051, filed on May 22, 1998.